The present invention relates to sintered bodies made of silicon carbide (hereafter, "sintered silicon carbide bodies") which can be used suitably not only as various structural engineering materials but also as electric materials, functional materials, etc. The present invention also relates to a process of producing such sintered bodies.
Sintered silicon carbide bodies are chemically stable both at room temperature and at high temperatures and have excellent mechanical strength at high temperatures and therefore they are expected to be useful as a construction material for producing various parts such as those for gas turbines, engines, heat exchangers, nozzles of burners, etc. These sintered silicon carbide bodies are also considered to be promising as a material for precision mold members such as optical disc, aspherical lens and the like since they have good properties such as surface smoothness, high thermal conductivity, wear resistance, and the like. In addition, sintered bodies made of highly pure silicon carbide, which are excellent in thermal resistance and chemical resistance, have been increasingly used for the production of boats and processing tubes according to the recent trend in the semiconductor industry in which higher temperatures are used for heat treatment.
Since silicon carbide is a substance which is of highly covalent bonding in nature and thus difficult to sinter, it is necessary to add to silicon carbide powder one or more elements selected from boron, carbon, aluminum, beryllium or their compounds as a sintering aid in an amount of several percents by weight in order to densify it so as to have a high density. Therefore, the sintered silicon carbide bodies obtained generally have electric resistivity which fluctuates greatly depending on the kind and amount of the sintering aid used. For example, it has been reported that sintered silicon carbide bodies which contain boron and carbon have an electric resistivity on the order of from 10.sup.4 to 10.sup.5 .OMEGA..multidot.cm and those which contain aluminum compounds have an electric resistivity on the order of from 10 to 10.sup.2 .OMEGA..multidot.cm. Both of them have relatively high electric resistivity.
To be in detail on the properties of such silicon carbides, it is noted that there are many polytypes in the crystals of thereof, which are roughly grouped into .alpha.-type and .beta.-type ones. Each of them exhibits semiconductivity. The semiconductivity of silicon carbide crystals can be of n-type or p-type depending on the kind and amount of impurities contained therein. There have been many reports on the electric resistivity of the silicon carbide crystals. For example, Busch described that .alpha.-type silicon carbide crystal has an electric resistivity of from 10.sup.-4 to 10.sup.-2 .OMEGA..multidot.cm at room temperature (cf. Silicon Carbide 1968, ed.by H. K. Henisch and R. Roy, Pergamon Press, New York (1969)). On the other hand, Nelson reported that .beta.-type one has an electric resistivity of from 10.sup.-2 to 10.sup.3 .OMEGA..multidot.cm at room temperature (cf. Silicon Carbide 1968. ed.by H. K. Henisch adn R. Roy, Pergamon Press, New York (1969)).
Generally, there is observed a tendency that the electric resistivity of single crystal decreases according as the concentration of impurities such as boron, nitrogen and the like contained therein increases. This is believed to be ascribable to the fact that the impurities act as carriers. Therefore, although it is true there is a possibility that single crystal silicon carbide synthesized under certain conditions may have an electric resistivity of not higher than 1 .OMEGA..multidot.cm, it is difficult to produce large single crystals of silicon carbide by mean of the conventional technology. Even in the case of producing small single crystals, there is a problem that the cost for their production is high.
In view of these technical or economical problems, it is considered at present most advantageous or effective to produce sintered bodies, which are aggregates of single crystals. However, this approach causes a new problem that since there are grain boundaries in the sintered bodies and presence of impurities, if any, therein results in increase in the electric resistivity. That is, since silicon carbide is rather difficult to sinter, it is necessary to use a sintering aid in order to obtain high density sintered bodies as described above. In this case, however, the sintering aid remains in the grain boundaries as an impurity or forms solid solution in the grains of silicon carbide. The sintering aid which remains in the grain boundaries behaves as an impurity, and there is a high possibility that the electric resistivity of the grain boundaries becomes higher than that of the respective single crystals.
Various developments have heretofore been made in order to obtain electrically conductive sintered silicon carbide bodies, and those methods which impart the sintered silicon carbide bodies with conductivity are roughly classified into the following groups.
(a) A method in which at least one electrically conductive substance is added to silicon carbide, and the electrically conductive substance is continuously brought in contact with the silicon carbide in the sintered bodies; PA1 (b) A method in which at least one electrically conductive substance or compound is added to silicon carbide so that the electrically conductive substances or compounds can be reacted with each other, or the silicon carbide can be reacted with the electrically conductive substance or compound, thus forming electrically conductive compound or complex phase in the grain boundaries of silicon carbide; PA1 (c) A method in which electrically conductive fibers are added to silicon carbide; and the like.
Among the compounds to be added, examples of the compounds used in the method (a) include TiC, ZrC, MoB.sub.2, ZrB.sub.2, MoSi.sub.2, TaSi.sub.2, ZrSi.sub.2, TiN and ZrN as disclosed in Japanese Patent Application (Kokai) No. Sho 58-209084. Examples of the compounds used in the method (b) include from 0.5 to 30% by weight of A1203 and Ti02 as described in Japanese Patent Application (Kokai) No. Sho 57-22173. Addition of from 0.5 to 30% by volume of at least one substance selected from the group consisting of carbides, nitrides, borides and oxides of elements belonging to the group IVa, Va and VIa of the periodic table, compounds of these and Al.sub.4 C.sub.3 is disclosed in Japanese Patent Application (Kokai) No. Sho 57-196770. Also, addition of from 1 to 10% by weight of at least one substance selected from aluminum and aluminum compounds, from 1 to 15% by weight of compounds of rare earth elements, and 8% by weight or less of a silicon compound is described in Japanese Patent Application (Kokai) No. Sho 60-195057. As for the method (c), there can be cited, for example, use of electrically conductive fibers composed of TiC or ZrB2 as disclosed in Japanese Patent Application (Kokai) NO.Sho 61-36162.
However, the methods for the production of the above-described electrically conductive sintered silicon carbide bodies involve the following problems.
That is, what is common among the methods (a), (b) and (c) above is the addition of at least one of electrically conductive substance or compound which is different in kind from silicon carbide, and therefore it is difficult to uniformly disperse such substance in the sintered bodies. Furthermore, there arises serious problem that when the substance is added, one or more of various characteristics such as high hardness, high corrosion resistance, high mechanical strength at high temperatures, high thermal conductivity, excellent surface smoothness, etc., which silicon carbide has inherently, will be deteriorated. It is impossible with the methods to obtain electrically conductive sintered silicon carbide bodies which satisfy the characteristic of the above-described sintered silicon carbide bodies and still have low electric resistivity.